Clarify meaning of training parameter in XGBoosterPredict()

include/xgboost/c_api.h

@@ -418,7 +418,12 @@ XGB_DLL int XGBoosterEvalOneIter(BoosterHandle handle,
  *          4:output feature contributions to individual predictions
  * \param ntree_limit limit number of trees used for prediction, this is only valid for boosted trees
  *    when the parameter is set to 0, we will use all the trees
- * \param training Whether the prediction value is used for training.
+ * \param training_dart_use_dropout Whether the prediction value is used for training under DART.
+ *    For most use cases, set it to 0. Set this parameter to 1 only if ALL of the following
+ *    conditions hold:
+ *    1) you are writing a training loop (inference doesn't apply);
+ *    2) you are using DART;
+ *    3) you are using a customized objective function.
  * \param out_len used to store length of returning result
  * \param out_result used to set a pointer to array
  * \return 0 when success, -1 when failure happens
@@ -427,7 +432,7 @@ XGB_DLL int XGBoosterPredict(BoosterHandle handle,
                              DMatrixHandle dmat,
                              int option_mask,
                              unsigned ntree_limit,
-                             int training,
+                             int training_dart_use_dropout,
                              bst_ulong *out_len,
                              const float **out_result);
 /*

src/c_api/c_api.cc

@@ -426,7 +426,7 @@ XGB_DLL int XGBoosterPredict(BoosterHandle handle,
                              DMatrixHandle dmat,
                              int option_mask,
                              unsigned ntree_limit,
-                             int32_t training,
+                             int training,
                              xgboost::bst_ulong *len,
                              const bst_float **out_result) {
   API_BEGIN();